Spongelike structures are beginning to show their worth in improving energy efficiency.
Already, the U.S. Department of Energy has developed a porous silicon that can replace graphite in lithium-ion batteries and therefore hold 10 times the charge of conventional cells. Now researchers at the Massachusetts Institute of Technology have developed a spongy mass of inexpensive graphite to generate steam.
Steam power has an exemplary, 300-year-old track record. And though it is no longer used in such modern machines as railroad engines, it's "important for desalination, hygiene systems, and sterilization," says Hadi Ghasemi, a postdoctoral researcher at MIT who led the research team.
"Especially in remote areas where the sun is the only source of energy," Ghasemi says, "if you can generate steam with solar energy, it would be very useful."
The spongy structure includes a layer of graphite flakes that lie over carbon foam. Because it's porous, it's light enough to float on water. Add a ray of sunshine and the sponge creates a "hotspot" in the graphite that draws up heated water through the pores, where it evaporates. The brighter the light, the more steam.
The foam resting on water in a beaker isn't necessarily a pretty sight, but it works hard. In fact it can convert 85 percent of solar energy into steam, a process far more efficient than current solar-powered steam generators. Further – again unlike conventional solar steam generators – it loses very little heat.
Current solar-powered generators require complex and expensive hardware to achieve a high concentration of sunlight to produce steam. That probably wouldn't be necessary with the MIT sponge system, according to a paper that Ghasemi and five colleagues report in the journal Nature Communications.
The work of the MIT team is part of an effort in energy research to improve the efficiency of what's called "solar harvesting." One approach has been to mix water with microscopic particles to form a nanofluid that rapidly absorbs solar energy. But that approach requires 1,000 times more solar energy than is available on the average sunny day.
The MIT sponge is 100 times more efficient. It needs only 10 times the amount of energy on a sunny day, the lowest amount needed so far to generate steam. "This is a huge advantage in cost-reduction," Ghasemi says.
Here's how it works: Steam is released from the surface of boiling water. So the MIT team decided to develop a material that would float on water. Eventually they developed a two-layered structure of energy-absorbent graphite flakes over the porous and buoyant carbon foam. The foam also insulates, keeping the heat from escaping the water.
Ghasemi's next step is to make the system even more efficient even with lower concentrations of solar energy for large steam generators. "There is still a lot of research that can be done on implementing this in larger systems," he says.